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Ground Layer AO at ESO’s VLT
Claire MaxInterim Director
UC ObservatoriesSeptember 14, 2014
Overview
• One VLT telescope devoted to wide fields and GLAO– Four sodium-layer laser guide stars– One adaptive secondary mirror feeds all AO
systems
• Two science instruments:– MUSE (24 visible-light IFUs)– HAWK-I (wide field near-IR imager)
• Each one has its own GLAO system– GALACSI AO system feeds MUSE (visible)– GRAAL AO system feeds HAWK-I (near-IR)
• Things to think about
VLT adaptive secondary: built by MicroGate, cost approx. $14M
MUSE: 24 visible light IFUs (!)
AO modules for these GLAO systems: large, sophisticated, complex
GALACSI design GALACSI on elevation bearing
MUSE +GALACSI AO: performance predictions
HAWK-I plus GRAAL AO:GLAO for near-IR wide field imaging
HAWK-I imager GRAAL GLAO system
GRAAL + HAWK-I: Performance predictions, K band
Image quality:No AO ~0.5”With AO ~0.4”
GRAAL + HAWK-I: Performance predictions, K band
About 6 arc min field
ESO built the ASSIST Test Stand to test AO systems with DM in the lab
Main Points
• Extremely ambitious ESO VLT wide field program– Both with and without GLAO
• Re-engineered adaptive secondary mirror (~$14M)
• Four sodium-layer LGS• Large and expensive instruments (MUSE, HAWK-
I) designed to take advantage of GLAO– MUSE (visible): 0.2 arc sec/px, HAWK-I (near-IR): 0.1 arc
sec/px– Low internal errors (?)
• Each instrument has its own AO module• Predictions:
– MUSE with GLAO: Image quality 0.65” -> 0.46” (30% improved)
– HAWK-I with GLAO: Image quality 0.50” -> 0.40” (20% improved)
Issues for extragalactic science with VLT GLAO
• What are/were the science trade-offs?
• Example: can trade field of view against image quality
– Wider field -> larger FWHM
• Wider field -> may be able to undertake larger surveys and/or use less telescope time for a given survey
• Larger FWHM -> lower SNR for given exposure time (so larger field may or may not speed up survey); less spatial resolution
• Trade depends on the science that you want to do
• I wasn’t able to find this kind of trade study in preparation for the two VLT GLAO systems + instruments
GRAAL- GALACSI Comparison
15
parameter GRAAL GALACSI
Instrument Hawk-I (IR imager) ESO Muse (VIS 3D-spectrograph) Lyon
Mode Maintenance mode GLAO Wide Field Mode Narrow Field Mode
Field of view 10” 7.5’ 1’ 7.5”
AO mode SCAO GLAO GLAO LTAO
Performance (S.R. ~ 80% in K-band) x1.7 EE gain x2 EE gain S.R. >5% (10% goal)
@650nm
Natural Guide Stars On axis, ~ 8 mag R-mag 14.5 within
6.7’ to 7.7’ radiusR-mag <17.5 within 52” to 105” radius
On Axis, NIR, Jmag 15Low Order sensing
Sky coverage Close to “bright” stars 95% >90% Science target =
TT reference
4LGSF config. NGS only Ø12’ Ø2’ Ø20”
WFS1 NGS L3-CCD(40*40 sub app.)
4 LGS L3-CCD (40*40 sub app.)1 TT L3-CCD
4 LGS L3-CCD(40*40 sub app.)1 TT L3-CCD
4 LGS L3-CCD(40*40 sub app.)1 IR Low Order
Loop frequency HO loop: ≥ 700 Hz HO loop: ≥ 700 HzTT loop: 250Hz
HO loop: 1 kHzTT loop: 200Hz
HO loop: 1 kHzLO loop: 200-500Hz